Light dispersion filter and optical module

ABSTRACT

A light dispersion filter is composed of three or more optically transparent layers each having a value equal to the value of the product of the refractive index and thickness of the optically transparent layer and transmitted light, and a plurality of partially reflective layers arranged alternately with the optically transparent layers and having predetermined reflectivities. Alternatively, a light dispersion filter has a plurality of etalon resonators which are arranged in series such that the value of the product of the refractive index of air and the interval of the etalon resonators is equal to the value of the product of the refractive index and thickness of the optically transparent layers.

TECHNICAL FIELD

The present invention relates to a light dispersion filter for applyingdesired dispersive properties to incident light and that hascharacteristics that a group delay amount varies depending on a lightwavelength.

BACKGROUND ART

In an optical communication system which employs optical fibers fortransmitting optical signals, it is known that optical signals dispersewithin optical fibers are effected by disturbed signal waveforms,resulting in limited transmission distances. The dispersion is aphenomenon attributable to a difference in the diffractive index fromone wavelength to another within an optical fiber that cause signals toarrive at different timings. Since an optical signal generally has apredetermined spectral width, a long transmission distance would causeits signal wave to extend on a time axis due to the dispersion,resulting in a failure to correctly receive information on the receiverside.

In basic networks, access networks and the like, light sources in a 1.55μm band are used because losses are generally minimized in this bandwithin an optical fiber. An optical signal within this wavelength bandreceives a dispersion value of approximately 17 ps/nm/km within anoptical fiber. Therefore, the influence of the dispersion can bealleviated during optical transmissions if the dispersive properties ofoptical fibers are shifted such that a dispersion value is minimized foroptical signals within the 1.55 μm wavelength band. An optical fiber,the dispersive properties of which are shifted from a predeterminedwavelength band, is called a “dispersion shift fiber” (DFS).

However, since the dispersion shift fiber has a complicated refractiveindex distribution in a cross-sectional direction as compared with anordinary optical fiber, the dispersive properties are directly affectedby distorted cores due to routing during the installation of opticalfibers, so that dispersion values vary depending on positions of opticalfibers. Also, the dispersion values largely fluctuate due to changes inthe ambient temperature. For these reasons, in conventional opticalcommunication systems which employ dispersion shift fibers, overalltransmission paths are designed in detail in a distributive strategy,but an increase in the distance and capacity of the transmission pathhas been impeded by higher cost of the dispersion shift fibers which aremanufactured based on the result of this design.

Generally, within an optical fiber, the product of the square of atransmission speed and a dispersion value of an optical signal isconstant. Therefore, if an optical signal has a spectral width, atransmittable distance is reduced in reciprocal proportion to the squareof the speed.

For example, when a directly modulated semiconductor laser is used as alight source, the dispersion exerts a large influence because of largefluctuations in the wavelength of the light source. On the other hand,when an externally modulated semiconductor laser is used as a lightsource, fluctuations in the wavelength can be limited by adjusting anexternal modulator of Mach-Zehnder type, so that a transmission speedcan be realized in a range of 10 to 40 Gb/s. However, even in thissituation, the influence of the dispersion cannot be eliminated.

Therefore, optical communication systems generally employ an approach ofincreasing a transmission distance by reducing the influence exerted bythe dispersion with the provision of a dispersion compensator disposedon the light source side or light receiver side for giving a dispersionwhich has properties opposite to those of dispersion given by opticalsignals within optical fibers.

As an example of providing such a dispersion compensator, aconfiguration which comprises a dispersion compensator in a transmissionunit (light source side) is disclosed in Patent Document 1(JP-10-242910-A). Also, as a specific configuration of a dispersioncompensator, Patent document 2 (JP-5-181028-A) and Patent document 3(JP-5-323391-A) each describe a dispersion compensator of a fiber ringresonator type. Further, a dispersion compensator of a fiber gratingresonator type described in Patent document 4 (JP-2000-252920-A), adispersion compensator of a multiple reflection delay plate typedescribed in Patent document 5 (Published Japanese Translation of PCTInternational Publication for Patent Application No. 2000-511655), andthe like are known as well.

Since these dispersion compensators employ a large scale configurationin order to compensate for the dispersion, the problem of an increase inthe cost of the optical communication system will occur. Nevertheless,these dispersion compensators are suitable for use in opticalcommunication systems for long distance transmissions because they arecapable of compensating for a large amount of dispersion (for example,3000 ps/nm).

On the other hand, as an attempt to reduce the size of the dispersioncompensator, Patent Document 6 (JP-6-160604-A), for example, describes adispersion compensator which utilizes an etalon resonator (hereinaftercalled the “etalon type dispersion compensator”). In the following, thisetalon type dispersion compensator will be briefly described.

The etalon resonator includes two partially transmitting mirrors(partially reflective layers) arranged with a predetermined spacingtherebetween to form a reflection type resonator, wherein peaks appearin the transmission property of light at intervals of a predeterminedfrequency called “FSR” (Free Spectral range). FSR is a frequencydetermined by the spacing interposed between the arranged partiallytransmitting mirrors. For example, in a configuration comprising anoptically transparent layer having a refractive index of 1.5, such asglass, and sandwiched by partially transmitting mirrors, FSR iscalculated to be approximately 100 GHz when the spacing between thepartially transmitting mirrors is 1 millimeter.

Incident light on the etalon resonator is transmitted through theresonator while repeating reflections between both end faces of the twopartially transmitting mirrors. In this event, while wavelength lighthaving more transparent components and wavelength light having lesstransparent components can be observed, any wavelength light reflects onthe end faces at an equal rate, so that it is thought that wavelengthlight having more transparent components reciprocates a larger number oftimes and is therefore transmitted through the etalon resonator with alarger delay. In other words, the dependency of the transmissionproperty on the wavelength means that a difference is produced in thedelay time depending on the wavelength. The etalon resonator can be usedas a light dispersion filter having large dispersive properties becauseits delay time largely varies in response to fluctuations in thewavelength. Among the dispersive properties of the etalon resonator, ifpart of the properties opposite to the dispersive properties of anoptical fiber is utilized to give dispersion to an optical signal whichenters into the optical fiber or an optical signal delivered aftertransmission in the optical fiber, it is possible to reduce theinfluence of the dispersion given by the optical fiber.

For the etalon type dispersion compensator, there is also known areflection type configuration which completely reflects light on one endface. When one end face is made to be completely reflective, light whichshould otherwise exit through the end face again passes along the etalonback to the incident side, so that the incident light is emitted fromthe same plane as the incident plane without loss. The amount ofdispersion experienced by the emitted light is the sum of the amount ofdispersion of the light which would otherwise be reflected, and theamount of dispersion of the light which would otherwise exit through theend face.

Since the etalon type dispersion compensator provides a substantiallyequal transmission property at every predetermined frequency interval(FSR), it can be used as a dispersion compensator for simultaneouslycompensating optical signals on multiple channels for dispersion, as inan optical wavelength division multiplexing (WDM).

However, conventional etalon type dispersion compensators can merelyaccomplish a dispersion compensation value of approximately 20 ps/nm,and the light dispersion filter has a transmittance of lower than 100%,thus causing an increase in the loss of optical power.

On the other hand, Patent Document 7 (JP-2000-105313-A) proposes amethod to increase a dispersion compensation value by using a pluralityof resonator type filters and expanding the bandwidth. Patent Document 7describes that the dispersive properties are improved by stacking threelayers of resonator filters each including a dielectric multi-layer filmhaving predetermined dispersive properties.

However, the configuration described in Patent Document 7 employs areflection type structure having a light incident plane, which alsoserves as an exit plane, in order to extract 100% of optical power whilecompensating for the dispersion, so that a device (circulator) isrequired for changing the optical path of incident light or emittedlight. This requirement results in a large configuration of the deviceon the light source side or light receiver side which has the dispersioncompensator, thus making the configuration unsuitable for a reduction insize. Further, since only dispersion is compensated for within apredetermined band, this configuration is not suited for multi-channelapplications such as the etalon type dispersion compensator described inPatent Document 6.

Thus, Non-Patent Document 1 (Moss Optical Fiber Conference, 2002Institute preprint manuscripts, TuT2, p133, FIG. 1) describes anexemplary modification to the configuration described in Patent Document7 for multi-channel applications. However, since the configurationdescribed in Non-Patent Document 1 is a reflection type, i.e., having alight incident plane which also serves as an exit plane, similar toPatent Document 7, it requires a circulator and the like, and istherefore not suited for enabling size reduction. Further, an increasednumber of parts, required for a mechanism for varying the amount ofdispersion, make the configuration complicated.

Patent Document 8 (Published U.S. patent application No. 2001-0021053)describes an exemplary configuration which is a similar configuration toNon-Patent Document 1 but does not employ a circulator. However, theconfiguration described in Patent Document 8 also has a problem similarto Non-Patent Document 1, and includes the difficulty of having anoptical communication system and an optical system that have anincreased size.

Problems of the conventional light dispersion filters described abovemay be summarized as follows:

A first problem is that the system configuration becomes large in anoptical communication system which has a conventional dispersioncompensator installed on the light source side or on receiver side.

This is because an optical path must be made sufficiently long in orderto delay an optical signal for purposes of accomplishing a largedispersion compensation value because the dispersion compensationnecessitates the creation of a delay circuit corresponding to awavelength component.

A second problem is that optical power is lost without any benefit inthe conventional dispersion compensators.

This is because a light coupling loss occurs when an optical signal isextracted from an optical fiber for performing the dispersioncompensation and the optical signal is again returned into the opticalfiber.

A third problem is that a dispersion compensation value is small in theconventional etalon type light dispersion filter.

This is because the conventional etalon type dispersion compensatorcannot increase the dispersion compensation value due to a short pathfor delaying light.

A fourth problem is that the system scale is large due to a complicatedoptical system in the conventional optical communication system whichcomprises a reflection type dispersion compensator.

This is because parts for switching optical paths are required since thereflective dispersion compensator has a light incident plane which alsoserves as an exit plane.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide a lightdispersion filter which is capable of serving as a dispersioncompensator for accomplishing a longer transmission path while achievinga reduction in size, power consumption and cost.

It is another object of the present invention to provide a lightdispersion filter which can be applied to a wide variety of pertinenttechnologies such as light dispersion measuring devices and the like,not limited to optical communications.

To achieve the above objects, a light dispersion filter according to thepresent invention comprises three or more optically transparent layerseach having a value equal to the value of the product of a refractiveindex and a thickness of the optically transparent layer, andtransmitted light, and a plurality of partially reflective layers eachhaving a predetermined reflectivity, and arranged alternately with theoptically transparent layers.

Alternatively, a light dispersion filter comprises a plurality of etalonresonators, each of which includes an optically transparent layer havinga value equal to the value of the product of a refractive index and athickness of the optically transparent layer, and transmitted light, andpartially reflective layers having predetermined reflectivities, andbonded to two surfaces of the optically transparent layer, respectively,wherein the etalon resonators are arranged in series such that the valueof the product of the refractive index of air and an interval of theetalon resonators is the value equal to the value of the product of therefractive index and thickness of the optically transparent layer.

In such configurations, since an effective optical path can be madelonger, a transmission time difference (dispersion) due to thewavelength can be made larger. It is therefore possible to provide adispersion compensator which, though small in size, exhibits a largedispersion compensation value.

Also, in the light dispersion filter according to the present invention,since similar transmission properties can be repeatedly presented atpredetermined frequency intervals (FSR) depending on the thickness ofthe optically transparent layer (the length of the resonator), thedispersion can be simultaneously compensated for in a plurality ofoptical signals at equal communication channel intervals, as in a WDMcommunication system, if FSR is set equal to the communication channelinterval.

On the other hand, an optical module according to the present inventioncomprises a transmission type light dispersion filter disposed on anoptical axis, which connects an optical active element to an opticalfiber, for compensating for dispersion given in the optical fiber.

Alternatively, an optical module comprises a reflection type light.dispersion filter for compensating for dispersion given in an opticalfiber, and comprises an optical active element, for use in opticalcommunications, disposed at a location deviated from an optical axiswhich connects the optical fiber to the light dispersion filter.

In such configurations, since desired dispersive properties can be givento an optical signal using the light dispersion filter on the opticalsignal transmitter side or receiver side, a transmission distance can beextended in an optical communication system using optical fibers,without installing a large-scaled dispersion compensator external to theoptical module. It is therefore possible to reduce the size and cost ofthe optical communication system.

Also, by containing the light dispersion filter, which serves as a lightdispersion compensator, in the optical module on the optical signaltransmitter side or receiver side, optical coupling loss can beminimized, without losing the optical power without any benefit, as isthe case with a conventional dispersion compensator.

Particularly, in an optical module which comprises a reflection typelight dispersion filter, since an optical active element is disposed ata location deviated from an optical axis which connects an optical fiberto a light dispersion filter, a circulator and the like are notrequired, so that an optical system can be simplified in configuration,thus preventing an increase in the size of the optical communicationsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating the configuration of afirst embodiment of a light dispersion filter according to the presentinvention;

FIG. 2A is a cross-sectional view illustrating an exemplaryconfiguration of a single-layer thin film for use in a multi-layer filmshown in FIG. 1;

FIG. 2B is a cross-sectional view illustrating an exemplaryconfiguration of a four-layer thin film for use in the multi-layer filmshown in FIG. 1;

FIG. 2C is a cross-sectional view illustrating an exemplaryconfiguration of an eight-layer thin film for use in the multi-layerfilm shown in FIG. 1;

FIG. 2D is a cross-sectional view illustrating an exemplaryconfiguration of a 12-layer thin film for use in the multi-layer filmshown in FIG. 1;

FIG. 3 is a graph showing the amount of dispersion and an operation bandwith respect to the number of etalon resonator layers of the lightdispersion filter illustrated in FIG. 1;

FIG. 4 is a graph showing the group delay characteristic of the lightdispersion filter illustrated in FIG. 1;

FIG. 5 is a cross-sectional view illustrating the configuration of asecond embodiment of the light dispersion filter according to thepresent invention;

FIG. 6 is a cross-sectional view illustrating the configuration of athird embodiment of the light dispersion filter according to the presentinvention;

FIG. 7 is a cross-sectional view illustrating the configuration of afourth embodiment of the light dispersion filter according to thepresent invention;

FIG. 8 is a cross-sectional view illustrating the configuration of afifth embodiment of the light dispersion filter according to the presentinvention;

FIG. 9 is a cross-sectional view illustrating the configuration of asixth embodiment of the light dispersion filter according to the presentinvention;

FIG. 10 is a cross-sectional view illustrating an exemplaryconfiguration of an optical module which employs a transmission typelight dispersion filter according to the present invention;

FIG. 11 is a cross-sectional view illustrating an exemplaryconfiguration of an optical module which employs a reflection type lightdispersion filter according to the present invention;

FIG. 12 is a cross-sectional view illustrating another exemplaryconfiguration of an optical module which employs the reflection typelight dispersion filter according to the present invention;

FIG. 13 is a cross-sectional view illustrating a first exemplarymodification to an optical module which employs the light dispersionfilter according to the present invention;

FIG. 14 is a cross-sectional view illustrating a second exemplarymodification to an optical module which employs the light dispersionfilter according to the present invention;

FIG. 15 is a block diagram illustrating an exemplary configuration of alight dispersion measuring device which employs the light dispersionfilter according to the present invention; and

FIG. 16 is a waveform chart showing the effect of a channel extractingmethod which employs the light dispersion filter according to thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

First, a description will be given of a light dispersion filter,according to the present invention, which is employed in a dispersioncompensator, optical modules, a light dispersion measuring device, acommunication channel extracting apparatus and the like, laterdescribed.

The aforementioned etalon resonator serves as a light dispersion filterwhich has an equal transmission property at every FSR interval. Withinthe etalon resonator, since incident light is transmitted therethroughafter it has reciprocated between partially reflective layers a largenumber of times, the etalon resonator has a long effective optical path,though small in size, and can therefore increase a wavelength-basedtransmission time difference (dispersion). A single etalon resonator maybe used to compensate for dispersion, like a conventional dispersioncompensator, but according to the calculation made by the inventors, theresulting dispersion compensation value is as small as 20 ps/nm atmaximum when an etalon resonator with FSR=100 GHz is used in a 20 GHzfrequency band.

In the present invention, a plurality of etalon resonators are adheredto each other to make up a light dispersion filter, thereby furtherincreasing an effective optical path of incident light. In this way, thedispersion compensation value can be further increased. However, simpleadhesion of etalon resonators cannot result in constant dispersiveproperties over a wide band. This is because there is a closerelationship between the dispersive properties for the wavelength andthe reflectivity between the respective etalon resonators.

In the present invention, the reflectivity is varied on each of theboundary planes between a plurality of etalon resonators, and thehighest reflectivity is set on a boundary plane near the center in thedirection of thickness of the light dispersion filter, while thereflectivity on the respective boundary planes is set to be graduallylower toward both end faces when a transmission type light dispersionfilter is configured. On the other hand, when a reflection type lightdispersion filter is configured, the lowest reflectivity is set on anincident plane, the reflectivity is set to 100% on the last end face onthe opposite side to the incident plane, and the reflectivity on therespective boundary planes is set to be gradually higher from theincident plane to the last end face. By thus setting the reflectivity oneach boundary plane, desired dispersive properties can be provided bythe entirety of the plurality of etalon resonators.

Incidentally, for designing a light dispersion filter, an extreme of thelight dispersion filter is generally designed to a desired value. Forexample, as described in Lenz, “Dispersive Properties of Optical Filtersfor WDM Systems”, IEEE Journal of Quantum Electronics, 1998, No. 34, vol8, pp. 1390-1402, since one etalon resonator has one extreme, amulti-layered etalon resonator, as in the present invention, will have atransmission property which includes extremes of the respective etalonresonators superimposed thereon.

In the light dispersion filter according to the present invention, theamount of dispersion is calculated by differentiating a group delay timefound from the superimposed extremes that are determined by therespective etalon resonators with respect to the wavelength. Also, inorder to ensure constant dispersive properties within an operating band(generally in a band corresponding to one-half period of FSR), theamount of dispersion is determined such that a group delaycharacteristic is linear in the band corresponding to one-half period ofFSR. A general designing method therefor is shown, for example, inJP-2002-122732-A. According to this official bulletin, a configurationhaving three or more extremes can provide a linear group delaycharacteristic in the band corresponding to one-half of FSR, and alarger slope (i.e., dispersion value) can be set for the group delaycharacteristic as the number of layers is increased. Therefore, thelight dispersion filter according to the present invention is preferablycomposed of three or more layers of etalon resonators.

The aforementioned JP-2002-122732-A describes in detail a reflectiontype dispersion compensator using an etalon resonator. Therefore, in adispersion compensator which utilizes an etalon resonator, it is knownthat its extreme (frequency at which the transmission property reaches apeak) is set to a desired value. However, the light dispersion filteraccording to the present invention differs from the aforementionedofficial bulletin in how values are set for the extremes.

In the present invention, extremes are set in the following manner forforming a transmission type light dispersion filter. A light dispersionfilter composed of three layers of etalon resonators will be given as anexample for the following description.

As mentioned above, the transmission property of each etalon resonatordepends on a respective extreme. Assume herein that respective extremesare represented by p1, p2, p3, and p1, p2, p3 are determined such thatconstant dispersive properties are derived in a band corresponding toone-half period of FSR.

Since the denominator of the transmission property of each etalonresonator is determined from the extreme, the denominator isproportional to:(1-p1/z)·(1-p2/z)·(1-p3/z)  (1)where z is a propagation constant for one round trip of incident lightwithin the etalon resonator.

Assume also that the reflectivities on respective boundary planes ofthree layers of etalon resonators (a first etalon resonator to a thirdetalon resonator) are represented by r0, r1, r2, r3, where r0 is thereflectivity on a boundary plane between the first etalon resonator andair; r1 on a boundary plane between the first etalon resonator and thesecond etalon resonator; r2 on a boundary plane between the secondetalon resonator and the third etalon resonator; and r3 on a boundaryplane between the third etalon resonator and air.

In this event, according to Equation 35 in Dowling, “Lightwave LatticeFilters for Optically Multiplexed Communication Systems”, IEEE Journalof Quantum Electronics, 1994, No. 12, vol. 3, pp. 471-486, thedenominator of the transmission properties of the etalon resonators isproportional to:r0·r3/z ³+(r0·r2+r1·r3+r0·r1·r2·r3)/z ²+(r0·r1+r1·r2+r2·r3)/z+1  (2)

Equation (1) expresses the transmission properties derived from theextremes, while Equation (2) expresses the transmission propertiesderived from the reflectivities on the respective boundary planes, andthese denominators must be matched in form. Therefore, the configurationcan be determined for a light dispersion filter used in the presentinvention if r0-r3 are determined such that the coefficient z is equalin Equation (1) and Equation (2).

While the number of conditions derived from the foregoing comparison ofthe coefficient is the same as the number of layers of the etalonresonators, there are a number of reflectivities larger by one valuethan the number of planes of the etalon resonators, i.e., the number ofconditions. Therefore, for r0-r3, when a value is arbitrary determinedfor a single one (r0 or r1 or r2 or r3), values are determined for theremaining ones. For example,

when p1=0.1×e^(iπ/6), p2=0.1×e^(-iπ/6), p3=0.25, if r0=0.1% isdetermined, this results in r1=2%, r2=80%, and r3=10%. Here, when r0 isset at a value close to nonreflection, it is understood that r1-r3 aresuch that the highest reflectivity is found on a boundary plane at thecenter of the light dispersion filter in the direction of thickness ofthe light dispersion filter, and the reflectivity becomes lower onboundary planes closer to both end faces. Note that the boundary planeon which the reflectivity is the highest is not always located at thecenter of the light dispersion filter, but may slightly move back andforth depending on the value of r0. Even when there are three or morelayers of etalon resonators, the respective reflectivities can be foundby comparing coefficients in the denominators of the transmissionproperties in a similar manner.

Further, for a reflection type light dispersion filter, thereflectivities are determined on respective boundary planes by comparingcoefficients between the transmission properties derived from extremesand the transmission properties derived from the reflectivities in amanner similar to the transmission type. In the reflection type lightdispersion filter, the lowest reflectivity is set on an incident plane,the reflectivity is set at 100% on the last end face on the oppositeside to the incident plane, and the reflectivity becomes graduallyhigher on respective boundary planes from the incident plane to the lastend face.

The light dispersion filter according to the present invention employs amulti-layered film composed of thin dielectric films as partiallyreflective layers of the respective etalon resonators, and sandwitchesthe multi-layered film by substrates (optically transparent layers) madeof glass, semiconductor or the like. Here, in regard to the fabricationof a transmission type light dispersion filter, the transmission powerhas a periodic property with respect to the frequency, like dispersiveproperties, so that the waveform of optical signals is likely to bedistorted. Therefore, the transmission power property of themulti-layered film preferably has the least possible dependence on thefrequency (dependency on the wavelength). As described above, in thepresent invention, the reflectivity of an arbitrary boundary plane in aplurality of etalon resonators can be set to a desired value, so thatthe reflectivity can be set higher on the light incident plane. In thisevent, because of the ability to reduce the optical power which returnsfrom the exit plane side of the light dispersion filter, the frequencydependency can be reduced for the dispersive properties of light whichis finally transmitted through the light dispersion filter. Forreference, when the reflectivity of light on the incident plane is setto 50% or higher, this effect appears prominent. When the reflectivityof light on the incident plane is set relatively high, the reflectivitygradually decreases on the respective boundary planes of the lightdispersion filter according to the present invention from the incidentplane to the exit plane. Alternatively, the reflectivity increases, onetime, after it decreases from the incident plane to the exit plane, andsubsequently decreases again toward the exit plane.

The light dispersion filter according to the present invention canfurther increase a dispersion compensation value, and can also extendthe operating band as the number of layers of the etalon resonators isincreased. However, an increase in the number of layers of the etalonresonators causes a reduction in the light transmission power due to anincreased number of reflective surfaces. While the number of layers ofthe etalon resonators is thought to be approximately 20 at maximum, anoptimal number of layers may be selected in consideration of the amountof attenuated transmission power and required dispersion properties.

For the light dispersion filter according to the present invention, thereflectivity is calculated for each of the boundary planes of the etalonresonators using the aforementioned Equations (1), (2), but thereflectivity of each boundary plane need not be strictly matched to thecalculated value. An error of approximately ±10% with respect to acalculated value is within an allowable range, and the transmissionproperty will not significantly lose its shape as long as therelationship among the reflectivities of the respective boundary planessatisfies the aforementioned conditions.

Also, the light dispersion filter according to the present invention canemploy an air layer instead of the substrate. However, since therespective etalon resonators must exhibit equal FSR, the product of thereflectivity and thickness of the material (substrate or air) of eachsubstrate should be set substantially equal. Here, while the product ofthe reflectivity and thickness of each medium is preferably equal,strict equality is not required. For example, assuming that eachsubstrate has a thickness of 1 mm, errors of several to several tens ofmicrons will not affect the transmission property.

Also, in the present invention, an adhesive (for example, an epoxy-basedadhesive) is used as a method of bonding the respective etalonresonators. This adhesive preferably has the same refractive index asthe substrate. The use of such an adhesive will eliminate reflection oflight on the boundary between the adhesive and the substrate, so thateven if the adhesive differs in thickness on the order of severalmicrons from one substrate to another, FSR's corresponding to thethicknesses (on the order of millimeters) of the respective substratesbecome equal, thus causing no problem.

Also, the light dispersion filter according to the present invention mayomit at least one multi-layered film which comprises the incident planeor exit plane. In this event, the reflectivity of the incident plane orexit plane is thought to be approximately 2% from the difference in therefractive index between the substrate and air.

Further, while the present invention employs a material such as adielectric material, such as glass, semiconductor, or the like for thesubstrate which determines the length of the etalon resonator, asemiconductor substrate including an amplifier circuit may be employedto amplify an optical signal which is transmitted therethrough.

Next, embodiments of the present invention will be described withreference to the drawings.

(First Embodiment)

FIG. 1 is a cross-sectional view illustrating the configuration of afirst embodiment of a light dispersion filter according to the presentinvention. While FIG. 1 illustrates an exemplary light dispersion filtercomposed of six layers of etalon resonators, the number of etalonresonators is not limited to six, but may be less than six (however,three or more) or more than six in accordance with desired dispersiveproperties and transmittance.

As illustrated in FIG. 1, the light dispersion filter of the firstembodiment comprises an alternating arrangement of first substrate 21 tosixth substrate 26 and first multi-layered film 27 to seventhmulti-layered film 33 which sandwich first substrate 21 to sixthsubstrate 26.

First substrate 21 to sixth substrate 26 and first multi-layered film 27to seventh multi-layered film 33 are bonded to each other using anadhesive. The adhesive preferably has a refractive index close to thatof first substrate 21 to sixth substrate 26 or first multi-layered film27 to seventh multi-layered film 33.

First substrate 21 to six substrate 26 are each formed in a thickness of1 mm such that FSR is 100 GHz, for example, using general glass whichhas the refractive index of 1.5.

First multi-layered film 27 to seventh multi-layered film 33 have lightreflectivities having different values, respectively. When forming atransmission type light dispersion filter, in the light dispersionfilter illustrated in FIG. 1, a higher reflectivity is set for amulti-layered film closer to a boundary plane near the center in thedirection of thickness of the light dispersion filter (fourthmulti-layered film 30 in FIG. 1), and lower reflectivities are set formulti-layered films closer to both end faces (first multi-layered film27 and seventh multi-layered film 33 in FIG. 1).

On the other hand, when forming a reflection type light dispersionfilter, the settings in the light dispersion filter illustrated in FIG.1 are such that the multi-layered film on the incident plane (firstmulti-layered film 27 in FIG. 1) has the lowest reflectivity, and thereflectivity becomes gradually higher toward the multi-layered film(seventh multi-layered film 33 in FIG. 1) of the last end face on theopposite side of the incident plane.

As illustrated in FIG. 2A, when forming a transmission type lightdispersion filter, first multi-layered film 27 and seventh multi-layeredfilm 33 are formed in a thickness which is one quarter of wavelength λ(=1.55 μm/refractive index 1.2) of transmitted light within the thinfilms, for example, by using a low-refractive index glass thin film(first thin film 40) having a refractive index of 1.2. In thisembodiment, the reflectivity between air and first substrate 21 is setto 0% by first multi-layered film 27, while the reflectivity between airand seventh substrate 26 is set to 0% by seventh multi-layered film 33.

While this embodiment shows an example of forming first multi-layeredfilm 27 and seventh multi-layered film 33 of single-layer thin films, itis also possible to employ low reflective films each composed of two orthree layers of thin films. In this event, the thickness of each thinfilm layer is not limited to one quarter of wavelength λ of thetransmitted light as long as low reflective film can be formed. In thetransmission type light dispersion filter of this embodiment, since ahigher light reflectivity is set for a multi-layered film closer to thecentral boundary plane, reflectivities of first multi-layered film 27and seventh multi-layered film 33 need not be limited to 0% as long asthey are not higher than the reflectivities of second multi-layer film28 and sixth multi-layered film 32. For example, since the reflectivityis 2% on the interface of air and first substrate 21 or sixth substrate26, first multi-layered film 27 and seventh multi-layered film 33 may beeliminated as long as the resulting light dispersion filter satisfiesthe aforementioned condition for the reflectivities.

On the other hand, when forming a reflection type light dispersionfilter, the reflectivity is set, for example, to 0% for only firstmulti-layered film 27. However, the reflection type is similar to thetransmission type in that the reflectivity of first multi-layered film27 need not be limited to the foregoing value, and the multi-layeredfilm may be composed of any number of layers as long as the resultingmulti-layered film satisfies the condition for the reflectivities.

As illustrated in FIG. 2B, when forming a transmission type lightdispersion filter, second muti-layered film 28 and sixth multi-layeredfilm 32 are each formed of a laminate film composed, for example, offour layers of thin films (first thin film 41 to fourth thin film 44).Here, SiO₂ (silicon dioxide) having a refractive index of 1.5 is usedfor first thin film 41 and third thin film 42, while TiO (titaniumoxide) having a refractive index of 2.8 is used for second thin film 43and fourth thin film 44. With such a composition, the refractive indexof second multi-layered film 28 and sixth multi-layered film 32 is setto 20%.

In this embodiment, the reflectivity of second multi-layered film 28 andsixth multi-layered film 32 are set to 20%, but since the lightreflectivity is only required to be set higher for a multi-layered filmcloser to the boundary plane at the center of the light dispersionfilter in the direction of thickness of the light dispersion filter, thereflectivity of second multi-layered film 28 and sixth multi-layeredfilm 32 need not be limited to 20% as long as it is not higher than thereflectivity of third multi-layered film 29 and fifth multi-layered film31. Also, second multi-layered film 28 and sixth multi-layered film 32are not limited to four layers as long as they satisfy theaforementioned condition for the reflectivities. Further, the thicknessof each thin film can also be freely designed as long as the resultingthin film satisfies the aforementioned condition for the reflectivities.

On the other hand, for forming a reflection type light dispersionfilter, four layers of thin films, for example, are used to set thereflectivity of second multi-layered film 28 to 20%. The reflection typeis similar to the transmission type in that the reflectivity of secondmulti-layered film 28 need not be limited to the foregoing value, andthe multi-layered film may be composed of any number of layers as longas the resulting multi-layered film satisfies the aforementionedcondition for the reflectivities.

As illustrated in FIG. 2C, when forming a transmission type lightdispersion filter, third multi-layered film 29 and fifth multi-layeredfilm 31 are each formed of a laminate film composed, for example, ofeight layers of thin films (first thin film 45 to eighth thin film 52).Here, SiO₂ (silicon dioxide) having a refractive index of 1.5 is usedfor first thin film 45, third thin film 46, fifth thin film 47, andseventh thin film 48, while TiO (titanium oxide) having a refractiveindex of 2.8 is used for second thin film 49, fourth thin film 50, sixththin film 51, and eighth thin film 52. With such a composition, therefractive index of third multi-layered film 29 and fifth multi-layeredfilm 31 is set to 40%.

The reflectivity of third multi-layered film 29 and fifth multi-layeredfilm 31 need not be limited to 40% as long as it is not higher than thereflectivity of fourth multi-layered film 30. Also, third multi-layeredfilm 29 and fifth multi-layered film 31 are not limited to eight layersas long as they satisfy the aforementioned condition for thereflectivities. Further, the thickness of each thin film can also befreely designed as long as the resulting thin film satisfies theaforementioned condition for the reflectivities.

On the other hand, when forming a reflection type light dispersionfilter, eight layers of thin films, for example, are used to set thereflectivity of third multi-layered film to 40%. The reflection type issimilar to the transmission type in that the reflectivity of thirdmulti-layered film 29 need not be limited to the foregoing value, andthe multi-layered film may be composed of any number of layers as longas the resulting multi-layered film satisfies the aforementionedcondition for the reflectivities.

As illustrated in FIG. 2D, fourth multi-layered film 30 is formed of alaminate film composed, for example, of 12 layers of thin films (firstthin film 53 to twelfth thin film 64). Here, SiO₂ (silicon dioxide)having a refractive index of 1.5 is used for first thin film 53, thirdthin film 54, fifth thin film 55, seventh thin film 56, ninth thin film57, and eleventh thin film 58, while TiO (titanium oxide) having arefractive index of 2.8 is used for second thin film 59, fourth thinfilm 60, sixth thin film 61, eighth thin film 62, tenth thin film 63,and twelfth thin film 64. With such a composition, the refractive indexof fourth multi-layered film 30 is set to 80%.

In the configuration of the transmission type light dispersion filter ofthis embodiment, since fourth multi-layered film 30 is at the positionof the central boundary plane, the reflectivity of fourth multi-layeredfilm 30 need not be limited to 80% as long as the highest reflectivitycan be set for fourth multi-layered film 30. Also, fourth multi-layeredfilm 30 need not be limited to 12 layers as long as it satisfies theaforementioned condition for the reflectivities. Further, the thicknessof each thin film can also be freely designed as long as the resultingthin film satisfies the aforementioned condition for the reflectivities.

On the other hand, for forming a reflection type light dispersionfilter, a thin film of ten layers, for example, is used to set thereflectivity of fourth multi-layered film 30 to 50%. The reflection typeis similar to the transmission type in that the reflectivity of fourthmulti-layered film 30 need not be limited to the foregoing value, andthe multi-layered film may be composed of any number of layers as longas the resulting multi-layered film satisfies the aforementionedcondition for the reflectivities. For the reflective type, for example,ten layers of thin films are used to set the reflectivity of fifthmulti-layered film 31 to 60%; 12 layers of thin films are used to setthe reflectivity of sixth multi-layered film 32 to 80%; and 12 layers ormore of thin films are used to set the reflectivity of seventhmulti-layered film 33 to 100%. Likewise, these multi-layered films neednot be limited in the number of layers as long as they satisfy theaforementioned condition for the reflectivities, and the thickness ofeach thin film can also be freely designed as long as the resulting thinfilm satisfies the aforementioned condition for the reflectivities.

In this embodiment, when a transmission type light dispersion filter isformed, the reflectivities exhibited by the multi-layered films are setto be symmetric about the central boundary plane of the light dispersionfilter toward both end faces, but the symmetric configuration is notessential as long as the condition can be met for providing the highestreflectivity on the boundary plane near the center.

Also, while this embodiment has shown an example in which firstsubstrate 21 to sixth substrate 26 are made of glass having the samerefractive index, first substrate 21 to sixth substrate 26 may be madeof different materials as long as they are composed such that theproduct of the refractive index and the thickness of each etalonresonator are equal so as to have an equal FSR result.

While this embodiment has shown a light dispersion filter composed ofsix layers of etalon resonators, an increase in the number of layers ofetalon resonators can further increase a dispersion compensation valueand extend the operating band, as shown in FIG. 3. As described above,an increase in the number of layers of etalon resonators results inreduced transmission power of light due to an increase in the number ofreflective planes. Therefore, an optimal number of layers may beselected in consideration of the amount of attenuated transmission powerand required dispersive properties.

As shown in FIG. 4, the light dispersion filter of this embodiment has agroup delay characteristic with peaks appearing every 100 GHz. Thedispersion is derived by differentiating the group delay characteristicof FIG. 4 with respect to the wavelength, and a dispersive property of300 psec/nm can be accomplished in a range of 30 GHz by utilizing partof the characteristics which rises to the upper right in the graph.Similarly, a dispersive property of −300 psec/nm can be accomplished ina range of 30 GHz by utilizing part of the characteristics which fallsto the lower right. This means that the light dispersion filter cancompensate for dispersion given within an optical fiber of approximately20 km in length.

(Second Embodiment)

As illustrated in FIG. 5, a light dispersion filter of a secondembodiment is composed of a plurality of etalon resonators (four in FIG.5), which are spaced apart from each other in a series arrangement.

As in the first embodiment, the etalon resonator is composed ofsubstrates (first substrate 71 to fourth substrate 74) made of adielectric material or semiconductor, and multi-layered films (firstmulti-layered film 75 to fourth multi-layered film 82) bonded tosandwich the substrates.

The substrate is formed in a thickness of 1 mm such that FSR is 100 GHz,for example, by using general glass which has a refractive index of 1.5.The multi-layered films are composed of laminated SiO₂ (silicon dioxide)thin films and TiO (titanium oxide) thin films, in a manner similar tothe first embodiment. In this embodiment, the respective etalonresonators are arranged at intervals of 1.5 mm such that FSR is 100 GHzcorresponding to the refractive index of air, i.e., 1.0.

Likewise, in the light dispersion filter of this embodiment, in the caseof a transmission type, the reflectivities of the multi-layered films ofthe respective etalon resonators are set such that the reflectivity isthe highest on the multi-layered film used by an etalon resonator nearthe center and is lower toward both ends. For example, the lightreflectivity of first multi-layered film 75 and eighth multi-layeredfilm 82 is set to 0%; the light reflectivity of second multi-layeredfilm 76 and seventh multi-layered film 81 to 20%; the light reflectivityof third multi-layered film 77 and sixth multi-layered film 80 to 40%;and the light reflectivity of fourth multi-layered film 78 and fifthmulti-layered film 79 to 80%.

On the other hand, in the case of a reflection type, the reflectivity isset to the lowest level (for example, 0%) on the incident plane (firstmulti-layered film 75 in FIG. 5), and the highest level (for example,100%) on the last end face (eighth multi-layered film 82 in FIG. 5) onthe opposite side to the incident plane.

Likewise, in this embodiment, the values of the reflectivities may beset in any way as long as the relationship of the reflectivities amongthe respective multi-layered films satisfies the aforementionedconditions, as is the case with the first embodiment.

A dispersion compensation value available by the transmission type lightdispersion filter of this embodiment was 400 psec/nm. This means thatthe light dispersion filter can compensate for dispersion that is givenin an optical fiber that is approximately 25 km in length.

(Third Embodiment)

FIG. 6 is a cross-sectional view illustrating the configuration of athird embodiment of the light dispersion filter according to the presentinvention.

As illustrated in FIG. 6, the light dispersion filter of the thirdembodiment is similar to the second embodiment in that a plurality ofetalon resonators (four in FIG. 6), spaced away from each other, arearranged in series, and gap substrates (first gap substrate 131 to thirdgap substrate 133) are inserted between the respective etalon resonatorsfor controlling the spacings of air layers. First gap substrate 131 tothird gap substrate 133 are formed of a material which is not opticallytransparent (for example, a metal or the like). The rest of theconfiguration is similar to the second embodiment, so that descriptionthereon is omitted.

According to the configuration of the light dispersion filter of thisembodiment, the distances between the etalon resonators, that constituteair layers, can be readily set using the gap substrates. With such aconfiguration, when a material having a large thermal expansioncoefficient is used for the gap substrates, changing the ambienttemperature can easily alter the transmission property and the amount ofdispersion.

(Fourth Embodiment)

As illustrated in FIG. 7, a light dispersion filter of a fourthembodiment is composed of a plurality of laminated etalon resonators(five layers in FIG. 7) in a manner similar to the first embodiment,wherein different materials are used for respective substrates (firstsubstrate 150 to fifth substrate 154 in FIG. 7) which determine thelengths of the associated etalon resonators, and they are directlyadhered to each other.

In this embodiment, for example, glass having a refractive index of 1.5is used for first substrate 150 and fifth substrate 154; ZnO (zincoxide) having a refractive index of 2.0 is used for second substrate 151and fourth substrate 153; and TiO (titanium oxide) having a refractiveindex of 2.8 is used for third substrate 152. Coating films 155, 156,having low reflectivity, may be formed on the surfaces of firstsubstrate 150 and fifth substrate 154 with air between the films.

Generally, reflection of light occurs on a boundary on which therefractive index of material changes. Therefore, when a plurality ofsubstrates, which are different in refractive index from one another,are prepared and adhere to each other as in this embodiment, the need toprovide a multi-layered film on the boundary planes between therespective substrates is eliminated. In this event, the reflectivitieson the boundary planes of the respective substrates are selected in amanner similar to the first embodiment, such that the reflectivity isthe highest on a boundary plane near the center of the light dispersionfilter in the direction of thickness of the light dispersion filter, andthe reflectivity becomes lower toward both ends. On the other hand, whena reflection type light dispersion filter is formed, the reflectivitiesof the respective substrates are selected such that the reflectivitybecomes gradually higher toward the last end face on the opposite sideof the light incident plane.

According to the configuration of the light dispersion filter of thisembodiment, the cost of the light dispersion filter can be reducedbecause of the elimination of the multi-layered films composing therespective etalon resonators and resulting elimination of materials forforming the multi-layered films, as well as the time and facilitiesrequired for manufacturing them.

(Fifth Embodiment)

As illustrated in FIG. 8, a light dispersion filter of a fifthembodiment comprises additional reflective mirror 160 for completelyreflecting light emitted from the light dispersion filter of the firstembodiment, in addition to the light dispersion filter of the firstembodiment.

Reflective mirror 160 is installed at a position at which the distanceto an exit plane multiplied by the value of the refractive index of amaterial (for example, air) between the exit plane and reflective mirror160 is equal to one half of the thickness of the substrate of the etalonresonator multiplied by the refractive index.

In such a configuration, an optical signal emitted from the lightdispersion filter is transmitted through the light dispersion filtertwice. For example, in the light dispersion filter of the firstembodiment illustrated in FIG. 1, when the light incident plane and thelast end face on the opposite side are made completely reflective, lightreflected toward the incident plane interferes with light reflected fromthe exit plane to reduce the dispersion compensation value in eachetalon resonator. By returning light from the exit plane using areflective mirror, as in this embodiment, a larger dispersioncompensation value can be accomplished than by a configuration in whichlight is completely reflected on the last end face of the etalonresonator.

(Sixth Embodiment)

As illustrated in FIG. 9, a light dispersion filter of a sixthembodiment employs semiconductors for a substrate of an etalonresonator, and has optical waveguide 162 and an optical amplifiercircuit for amplifying an optical signal formed on semiconductorsubstrate 161.

The optical amplifier circuit comprises a plurality of diffractiongratings (first diffraction grating 163 to fifth diffraction grating 167in FIG. 9) formed at each predetermined distance on the opticalwaveguide, for example, utilizing known stimulated emission light.Coating films 168, 169 having a low reflectivity may be formed on theinterface of semiconductor substrate 161 with air. Since the diffractiongrating exhibits a higher reflectivity because there are a larger numberof periods of gratings, a transmission type light dispersion filter iscreated when the reflection is set highest for a diffraction gratingnear the center of optical waveguide 162 (third diffraction grating inFIG. 9), and the reflectivity is set to be lower for diffractiongratings that are closer to both end faces. On the other hand, areflection type light dispersion filter is created when the number ofperiods is set smallest for first diffraction grating 163 on the lightincident plane side, and the number of periods is set largest for fifthdiffraction grating 167, and a high reflectivity (for example, 100%) isset for multi-layer films disposed on the incident plane and on the lastend on the opposite side.

According to the light dispersion filter of this embodiment, sincesemiconductors, the refractive index of which is approximately 3.5, areused for the substrate of the etalon resonators, the device length canbe formed shorter than the configuration which employs glass having arefractive index of 1.5. Also, since the light dispersion filter has anoptical amplifying function, the light dispersion filter can compensatefor a coupling loss with an optical fiber.

(Seventh Embodiment)

A seventh embodiment proposes an optical module which employs atransmission type light dispersion filter from among the lightdispersion filters described in the first embodiment to the sixthembodiment.

As illustrated in FIG. 10, optical module 1 of the seventh embodimentcontains, for example, transmission type light dispersion filter 3described in the first embodiment to the sixth embodiment, and useslight dispersion filter 3 as a dispersion compensator for compensatingfor dispersion by optical fiber 4.

When the optical module is a transmitter module for transmitting anoptical signal, optical module 1 comprises optical active element 2which serves as a light source, applies light emitted from opticalactive element 2 with dispersion that is the reverse to the dispersionin optical fiber 4 by using light dispersion filter 3 according to thepresent invention, and introduces the resulting light into optical fiber4.

On the other hand, when the optical module is a reception module forreceiving an optical signal, optical module 1 comprises optical activeelement 2 which serves as a light receiving element, applies an opticalsignal transmitted through optical fiber 4 with dispersion that is thereverse to the dispersion in optical fiber 4 by using light dispersionfilter 3 according to the present invention, and introduces theresulting optical signal into optical active element 2.

In the optical module of this embodiment, since light dispersion filter3 is of the transmission type, light dispersion filter 3 is disposed onthe optical axis which connects optical fiber 4 to optical activeelement 2.

While optical active element 2, which is a light source or a lightreceiving element, and light dispersion filter 3 are only described inoptical module 1 illustrated in FIG. 10, optical module 1 may alsocomprise a lens for converging light emitted from optical active element2 onto light dispersion filter 3 and a lens for converging lighttransmitted through light dispersion filter 3 into the core of opticalfiber 4. Alternatively, optical module 1 may comprise a lens forconverging light emitted from optical fiber 4 onto light dispersionfilter 3, and a lens for converging light transmitted through lightdispersion filter 3. Further, an optical isolator may be disposedbetween the light source and light dispersion filter 3 in order toprevent light reflected from light dispersion filter 3 from returning tothe light source to disturb oscillations.

Since the light dispersion filter of this embodiment can be reduced insize as compared with conventional dispersion compensators, the lightdispersion filter can be contained, for example, in an optical modulewhich includes a directly modulated semiconductor laser, which serves asa light source, an externally modulated semiconductor laser, and thelike. Consequently, since it is possible to provide dispersiveproperties for compensating for dispersion in an optical fiber withinthe optical module, an optical-fiber based optical transmission systemcan extend the transmission distance without providing a large sizedispersion compensator external to the optical module.

(Eighth Embodiment)

An eighth embodiment proposes an optical module which employs areflection type light dispersion filter from among the light dispersionfilters described in the first embodiment to sixth embodiments.

As illustrated in FIG. 11, optical module 11 of the eighth embodimentcontains, for example, reflection type light dispersion filter 13described in the first embodiment to sixth embodiment, and uses lightdispersion filter 13 as a dispersion compensator for compensating fordispersion by optical filter 14.

When the optical module is a transmitter module for transmitting anoptical signal, optical module 11 comprises optical active element 12which serves as a light source, applies light emitted from opticalactive element 12 with dispersion that is the reverse to the dispersionin optical fiber 14 by using light dispersion filter 13 according to thepresent invention, and then introduces the resulting light into opticalfiber 14.

On the other hand, when the optical module is a receiver module forreceiving an optical signal, optical module 11 comprises optical activeelement 12 which serves as a light receiving element, applies an opticalsignal transmitted through optical fiber 14 with dispersion that is thereverse to the dispersion in optical fiber 14 by using light dispersionfilter 13 according to the present invention, and then introduces theresulting optical signal into optical active element 12.

In the optical module of this embodiment, since light dispersion filter13 is of the reflection type, the optical active element is placed at alocation deviated from the optical axis which connects optical fiber 14to light disperse filter 13. The placement of optical active element 12at such a location eliminates the need for an element (circulator) thatwill change the optical path of incident light or emitted light, thusmaking it possible to prevent an increase in the size of the opticalmodule.

The optical module of this embodiment may also comprise a lens forconverging light emitted from optical active element 12 onto lightdispersion filter 13, and lens for converging light reflected by lightdispersion filter 13 into the core of optical fiber 14. Alternatively,the optical module may comprise a lens for converging light emitted fromoptical fiber 14 onto light dispersion filter 13, and a lens forconverging light reflected by light dispersion filter 13 onto opticalactive element 12. Further, an optical isolator may be disposed betweenthe light source and the light dispersion filter in order to preventlight reflected from light dispersion filter 13 from returning to thelight source to disturb oscillations.

As in the seventh embodiment, the optical module of this embodiment canbe contained, for example, in an optical module which includes adirectly modulated semiconductor layer, which serves as a light source,an externally modulated semiconductor laser, and the like. Consequently,since it is possible to provide dispersive properties for compensatingfor dispersion in an optical fiber within an optical module, anoptical-fiber based optical transmission system can extend thetransmission distance without providing a large sizd dispersioncompensator external to the optical module.

(Ninth Embodiment)

A ninth embodiment proposes another exemplary configuration of anoptical module which employs a reflection type light dispersion filterfrom among the light dispersion filters described in the first to sixthembodiments.

As illustrated in FIG. 12, optical module 100 of this embodimentcontains, for example, reflection type light dispersion filter 102described in the first embodiment to sixth embodiment, and uses lightdispersion filter 102 as a dispersion compensator for compensating fordispersion due to optical filter 104.

When the optical module is a transmitter module for transmitting anoptical signal, optical module 100 comprises optical active element 101which serves as a light source, applies light emitted from opticalactive element 101 with dispersion that is the reverse to the dispersiondue to optical fiber 104 by using light dispersion filter 102 accordingto the present invention, and then introduces the resulting light intooptical fiber 104.

On the other hand, when the optical module is a receiver module forreceiving an optical signal, optical module 100 comprises optical activeelement 101 which serves as a light receiving element, applies anoptical signal transmitted through optical fiber 104 with dispersionthat is the reverse to the dispersion in optical fiber 104 by usinglight dispersion filter 102 according to the present invention, and thenintroduces the resulting optical signal into optical active element 101.

Further, optical module 100 of this embodiment comprises half mirror 103disposed on the optical axis which connects optical fiber 104 to lightdispersion filter 102, and disposes optical active element 101 on theoptical axis of light reflected from half mirror 103.

In such a configuration, since the optical signal passes through halfmirror 103 twice, light emitted from optical active element 101 or lightreceived through optical fiber 104 is reduced in power by a factor offour.

However, since half mirror 103 is small enough to be accommodated withinoptical module 100, unlike a circulator and the like, half mirror 103can be contained in an optical module which includes a directlymodulated semiconductor laser which serves as a light source, anexternally modulated semiconductor laser, and the like. Thus, like theseventh and eighth embodiments, since it is possible to providedispersive properties for compensating for dispersion in an opticalfiber within the optical module, an optical-fiber based opticaltransmission system can extend the transmission distance withoutproviding a large sized dispersion compensator external to the opticalmodule.

(Tenth Embodiment)

A tenth embodiment is an exemplary modification to the optical moduledescribed in the seventh embodiment to ninth embodiment.

As illustrated in FIG. 13, an optical module of the tenth embodimentcomprises first temperature controller 93 for controlling thetemperature of the optical active element; and second temperaturecontroller 94 for controlling the temperature of the light dispersionfilter, for example, in the optical module illustrated in the seventhembodiment to ninth embodiment. First temperature controller 93 andsecond temperature controller 94 each comprise, for example, a heater,not shown; a current source for supplying a current to the heater; atemperature sensor for detecting the temperature of the optical activeelement or light dispersion filter; and a controller for turning ON/OFFthe current supplied to the heater in accordance with a detected valueof the temperature sensor.

While FIG. 13 illustrates the configuration of the optical module havinga transmission type light dispersion filter illustrated in FIG. 7, givenas an example, this embodiment can also be applied to a reflection typelight dispersion filter as shown in the eighth and ninth embodiments.

Generally, a temperature changing rate of a wavelength corresponding toan optical active element is approximately 0.08 nm/° C., while a lightdispersion filter exhibits a temperature changing rate of 0.01 nm/° C.Therefore, by independently controlling the temperatures of the opticalactive element and light dispersion filter using first temperaturecontroller 93 and second temperature controller 94, they can each bemodified to have desired properties. It should be noted that a change intemperature causes the light dispersion filter to change the amount ofdispersion together with the wavelength. Since changing the amount ofdispersion depends on the material, the number of layers, and the likeused in a particular light dispersion filter, a light dispersion filtermay be designed to present optimal values.

(Eleventh Embodiment)

An eleventh embodiment is another exemplary modification to the opticalmodule described in the seventh embodiment to tenth embodiment.

As illustrated in FIG. 14, the eleventh embodiment is configured suchthat an optical fiber can be removed from optical module 110 whichcontains, for example, a light dispersion filter according to thepresent invention illustrated in the seventh to tenth embodiments.Optical connector 113, for example, is fixed at an optical fiberreceptacle. While FIG. 14 illustrates the optical module having thetransmission type light dispersion filter described in the seventhembodiment, given as an example, this embodiment can also be applied toa configuration which has a reflection type light dispersion filter asin the eighth and ninth embodiments, and to a configuration having afirst and a second temperature controllers as in the tenth embodiment.

In such a configuration, since the cost of optical module 110 can bereduced, an optical module can be provided that is suitable for use inlow-end devices.

(Twelfth Embodiment)

A twelfth embodiment proposes a light dispersion measuring device whichapplies the light dispersion filter described in the first embodiment tosixth embodiment.

As illustrated in FIG. 15, the twelfth embodiment is a light dispersionmeasuring device for measuring the amount of dispersion, for example, byusing the light dispersion filter according to the present inventionillustrated in the first embodiment to sixth embodiment.

The light dispersion measuring device comprises optical demultiplexer124 for branching an optical signal; light dispersion filter 121 throughwhich one of the optical signals branched by optical demultiplexer 124passes; first light receiver 122 for generating an electric signalcorresponding to the optical signal which has passed through lightdispersion filter 121; second light receiver 123 for generating anelectric signal corresponding to another optical signal branched byoptical demultiplexer 124; and signal differential circuit 124 forgenerating the difference between signals generated by first lightreceiver 122 and second light receiver 123.

In such a configuration, since the output value of signal differentialcircuit 125 is proportional to a dispersion value by light dispersionfilter 121, it is possible to find the amount of dispersion of lightdispersion filter 121. Also, when the amount of dispersion from lightdispersion filter 121 is known beforehand, the amount of dispersion foran optical signal applied to the light dispersion measuring device canbe measured from the difference between the amount of dispersion fromlight dispersion filter 121 and the amount of dispersion from outputvalue of signal differential circuit 125.

(Thirteenth Embodiment)

A thirteenth embodiment proposes a communication channel extractingapparatus which applies the light dispersion filter described in thefirst embodiment to sixth embodiment.

Generally, the frequency interval of each communication channel in a WDMoptical communication system is normalized to 50 GHz, 100 GHz, and thelike.

As described above, the transmission type light dispersion filteraccording to the present invention has a frequency region for every FSRperiod in which light is most transmitted through the light dispersionfilter. Therefore, as shown in FIG. 16, by setting FSR of the lightdispersion filter according to the present invention, illustrated in thefirst embodiment to six embodiment, for example, to be smaller than thechannel interval of a WDM optical communication system, the lightdispersion filter can be used as a communication channel extractingapparatus for selecting a communication channel light of the WDM opticalcommunication system, which is transmitted through the light dispersionfilter.

Specifically, the FSR of the light dispersion filter is extended by 10to 20% from the channel interval, for example, such as 60 GHz or 120GHz. In this event, when the temperature of the light dispersion filteris changed using the temperature controller shown in the tenthembodiment, the light dispersion filter changes in the FSR, so that onlyan optical signal on a desired communication channel can be extractedfrom a plurality of communication channels possessed by the WDM opticalcommunication system.

1. A light dispersion filter for applying desired dispersion to anincident optical signal, comprising: three or more optically transparentlayers each having a value equal to the value of a product of arefractive index and a thickness of said optically transparent layer,and transmitting light; and a plurality of partially reflective layershaving predetermined reflectivities, and arranged alternately with saidoptically transparent layers, wherein the reflectivity is highest on apartially reflective layer disposed near the center of said lightdispersion filter in a direction of thickness of said light dispersionfilter, and the reflectivities of the respective partially reflectivelayers are gradually lower toward both end faces of said lightdispersion filter.
 2. A light dispersion filter for applying desireddispersion to an incident optical signal, comprising: a plurality ofetalon resonators, each including: an optically transparent layer havingan equal value of a product of a refractive index and a thickness, andtransmitting light; and partially reflective layers having predeterminedreflectivities, and bonded to two surfaces of said optically transparentlayer, respectively, wherein said etalon resonators are arranged inseries such that the value of the product of the refractive index of airand an interval of said etalon resonators is equal to the value of theproduct of the refractive index and thickness of said opticallytransparent layer.
 3. The light dispersion filter according to claim 2,wherein the reflectivity is the highest on a partially reflective layerdisposed near a center of said light dispersion filter in a direction ofthickness of said light dispersion filter, and the reflectivities of therespective partially reflective layers are gradually lower toward bothend faces of said light dispersion filter.
 4. The light dispersionfilter according to claim 1, further comprising a reflective mirror forcompletely reflecting light, said reflective mirror being disposed at alocation at which the value of a product of a distance to a light exitplane and the refractive index of a material between said exit plane andsaid reflective mirror is one-half of a product of the refractive indexand thickness of said optically transparent layer.
 5. The lightdispersion filter according to claim 3, further comprising a reflectivemirror for completely reflecting light, said reflective mirror beingdisposed at a location at which the value of a product of a distance toa light exit plane and the refractive index of a material between saidexit plane and said reflective mirror is one-half of a product of therefractive index and thickness of said optically transparent layer.
 6. Alight dispersion filter for applying desired dispersion to an incidentoptical signal, comprising: three or more optically transparent layerseach having a value equal to the value of the product of a refractiveindex and a thickness of said optically transparent layer, andtransmitting light; and a plurality of partially reflective layershaving predetermined reflectivities, and arranged alternately with saidoptically transparent layers, wherein the reflectivities of saidpartially reflective layers are gradually higher from a light incidentplane side for said light dispersion filter to a last end face on theopposite side to said incident plane.
 7. The light dispersion filteraccording to claim 2, wherein the reflectivities of said partiallyreflective layers are gradually higher from a light incident plane sidefor said light dispersion filter to a last end face on the opposite sideto said incident plane.
 8. The light dispersion filter according toclaim 1, wherein: said optically transparent layer is a dielectricsubstrate; and said partially reflective layer is a thin film or amulti-layered film composed of a plurality of laminated thin films. 9.The light dispersion filter according to claim 2, wherein: saidoptically transparent layer is a dielectric substrate; and saidpartially reflective layer is a thin film or a multi-layered filmcomposed of a plurality of laminated thin films.
 10. The lightdispersion filter according to claim 6, wherein: said opticallytransparent layer is a dielectric substrate; and said partiallyreflective layer is a thin film or a multi-layered film composed of aplurality of laminated thin films.
 11. The light dispersion filteraccording to claim 1, wherein: said optically transparent layer is asemiconductor substrate; and said light dispersion filter compriseslight amplifying means in said semiconductor substrate for amplifying anincident optical signal.
 12. The light dispersion filter according toclaim 2, wherein: said optically transparent layer is a semiconductorsubstrate; and said light dispersion filter comprises light amplifyingmeans in said semiconductor substrate for amplifying an incident opticalsignal.
 13. The light dispersion filter according to claim 6, wherein:said optically transparent layer is a semiconductor substrate; and saidlight dispersion filter comprises light amplifying means in saidsemiconductor substrate for amplifying an incident optical signal. 14.The light dispersion filter according to claim 1, wherein said opticallytransparent layers and said partially reflective layers are bonded by anadhesive having the same refractive index as said optically transparentlayers.
 15. The light dispersion filter according to claim 2, whereinsaid optically transparent layers and said partially reflective layersare bonded by an adhesive having the same refractive index as saidoptically transparent layers.
 16. The light dispersion filter accordingto claim 6, wherein said optically transparent layers and said partiallyreflective layers are bonded by an adhesive having the same refractiveindex as said optically transparent layers.
 17. An optical modulecomprising: an optical active element for use in optical communications;an optical fiber serving as an optical signal transmission medium; and atransmission type light dispersion filter disposed on an optical axisconnecting said optical active element to said optical fiber forcompensating for dispersion in said optical fiber.
 18. An optical modulecomprising: an optical active element for use in optical communications;an optical connector for removably fixing an optical fiber serving as anoptical signal transmission medium; and a transmission type lightdispersion filter disposed on an optical axis connecting said opticalactive element to said optical fiber for compensating for dispersion insaid optical fiber.
 19. An optical module comprising: an optical fiberserving as an optical signal transmission medium; a reflection typelight dispersion filter for compensating for dispersion in said opticalfiber; and an optical active element disposed at a location deviatedfrom an optical axis connecting said optical fiber to said lightdispersion filter, for use in optical communications.
 20. An opticalmodule comprising: an optical connector for removably fixing an opticalfiber serving as an optical signal transmission medium; a reflectiontype light dispersion filter for compensating for dispersion given insaid optical fiber; and an optical active element disposed at a locationdeviated from an optical axis connecting said optical fiber to saidlight dispersion filter, said optical active element being for use inoptical communications.
 21. The optical module according to claim 19,further comprising: a half mirror positioned on the optical axisconnecting said optical fiber to said light dispersion filter, whereinsaid optical active element is disposed on an optical axis of lightreflected from said half mirror.
 22. The optical module according toclaim 20, further comprising: a half mirror positioned on the opticalaxis connecting said optical fiber to said light dispersion filter,wherein said optical active element is disposed on an optical axis oflight reflected from said half mirror.
 23. The optical module accordingto claim 17, further comprising: a first temperature controller forcontrolling the temperature of said optical active element; and a secondtemperature controller for controlling the temperature of said lightdispersion filter.
 24. The optical module according to claim 18, furthercomprising: a first temperature controller for controlling thetemperature of said optical active element; and a second temperaturecontroller for controlling the temperature of said light dispersionfilter.
 25. The optical module according to claim 19, furthercomprising: a first temperature controller for controlling thetemperature of said optical active element; and a second temperaturecontroller for controlling the temperature of said light dispersionfilter.
 26. The optical module according to claim 20, furthercomprising: a first temperature controller for controlling thetemperature of said optical active element; and a second temperaturecontroller for controlling the temperature of said light dispersionfilter.
 27. The optical module according to claim 17, wherein saidoptical active element is a light source for emitting an optical signal.28. The optical module according to claim 18, wherein said opticalactive element is a light source for emitting an optical signal.
 29. Theoptical module according to claim 19, wherein said optical activeelement is a light source for emitting an optical signal.
 30. Theoptical module according to claim 20, wherein said optical activeelement is a light source for emitting an optical signal.
 31. Theoptical module according to claim 17, wherein said optical activeelement is a light receiving element for receiving an optical signal.32. The optical module according to claim 18, wherein said opticalactive element is a light receiving element for receiving an opticalsignal.
 33. The optical module according to claim 19, wherein saidoptical active element is a light receiving element for receiving anoptical signal.
 34. The optical module according to claim 20, whereinsaid optical active element is a light receiving element for receivingan optical signal.
 35. A light dispersion measuring device comprising:an optical demultiplexer for branching an optical signal; the lightdispersion filter according to claim 1, through which one of the opticalsignals branched by said optical demultiplexer passes; a first lightreceiver for generating an electric signal corresponding to the opticalsignal which has passed through said light dispersion filter; a secondlight receiver for generating an electric signal corresponding toanother optical signal branched by said optical demultiplexer; and asignal differential circuit for generating a difference between thesignals generated from said first light receiver and said second lightreceiver.
 36. A light dispersion measuring device comprising: an opticaldemultiplexer for branching an optical signal; the light dispersionfilter according to claim 2, through which one of the optical signalsbranched by said optical demultiplexer passes; a first light receiverfor generating an electric signal corresponding to the optical signalwhich has passed through said light dispersion filter; a second lightreceiver for generating an electric signal corresponding to anotheroptical signal branched by said optical demultiplexer; and a signaldifferential circuit for generating a difference between the signalsgenerated from said first light receiver and said second light receiver.37. A light dispersion measuring device comprising: an opticaldemultiplexer for branching an optical signal; the light dispersionfilter according to claim 6, through which one of the optical signalsbranched by said optical demultiplexer passes; a first light receiverfor generating an electric signal corresponding to the optical signalwhich has passed through said light dispersion filter; a second lightreceiver for generating an electric signal corresponding to anotheroptical signal branched by said optical demultiplexer; and a signaldifferential circuit for generating a difference between the signalsgenerated from said first light receiver and said second light receiver.38. A communication channel extracting apparatus comprising a lightdispersion filter through which optical signals on a plurality ofcommunication channels set at predetermined frequency intervals pass,said light dispersion filter having a free spectral range set wider thanthe frequency interval.
 39. The communication channel extractingapparatus according to claim 38, further comprising a temperaturecontroller for controlling the temperature of said light dispersionfilter.